Radio frequency probe systems are known and are useful for detecting the presence or absence of flowable materials, so that such systems may be employed to provide bin level indicators and control systems, tank level indicators, and material detection and control systems. Radio frequency capacitance probe systems are particularly useful for detecting difficult materials, which are difficult to detect by the use of other types of material detectors, such as devices using rotatable paddles, upon which the material imposes drag, or pressure detectors, which respond to the pressure or weight of the material. Other difficult materials can do damage to other types of material detectors. Radio frequency capacitance probes can be built in such a manner that they are relatively invulnerable to damage. Some examples of such difficult materials are powders, such as plastic powders, slaked lime, carbon fluff, iron oxide, general dust, sugar, flour and many others; granules, such as plastic pellets, metal pellets, chemical fertilizers, grains and many others; lumps, such as coal, caustic lime, ore, gravel and many others; liquids, such as fuel, solvents, acids, alkali, liquified gas, distilled water and others; mixed liquids, such as factory waste, sewage, slurries and many others; foam, such as latex foam, beer foam, soap foam and others; viscous materials, such as mud, sludge, tar, paint, grease, adhesive paste, soap, paraffin, cheese, honey and others; and interfaces between materials, such as oil and water, mercury and water, alcohol and acid, sediment in water, liquid and foam, and others.
Various methods and devices have been used for measuring or detecting the electrical capacitance of an electrode or probe in material detection systems. One method has been to connect the capacitance of the probe into a radio frequency oscillator circuit, in such a manner that the oscillating frequency of the oscillator circuit is changed by changes in the capacitance of the probe. The frequency of the radio frequency oscillations is then measured, and changes are used to actuate indicating devices or a control system. In certain material detection systems, the frequency is measured by converting the radio frequency signals into pulses, counting the pulses electronically, and interpreting the counted results with the aid of a microprocessor. Another method has been to connect the capacitance of the probe into a radio frequency oscillator circuit in such a manner that the increasing capacitance of the probe causes the oscillator to commence oscillating.
Many difficult materials cause a problem in that they cling or adhere to the probe or electrode, so that the adherent material changes the initial capacitance of the probe. When the probe has just been installed, and has not been touched by the material, the electrical capacitance of the typical small probe is quite low, perhaps on the order of one picofarad (pf). When the material contacts or surrounds the probe, the capacitance is materially increased, and this increase is detected and used to change the state of an indicator or a control system. When the material recedes or moves out of contact with the probe, some of the material clings or adheres to the probe. As a result, the capacitance of the probe does not return to its initial low value, but rather decreases to a somewhat higher value than the initial value. Thus, it can be said that the calibration of the probe has been changed by the adherent material. The capacitance value of the probe when material is present around the probe is still substantially higher than the capacitance value when the body of the material is absent from the probe, but the material-absent value of the capacitance is higher than the initial capacitance value of the probe. This change in the calibration of the probe creates a need to recalibrate the system which is employed to differentiate between the material-present value and the material-absent value of the capacitance. If the system is too sensitive, it may experience difficulties in differentiating reliably between the material-present and the material-absent capacitance values of the probe. Thus, a need arises to provide a system having a variable or adjustable sensitivity.